Re:[PATCH v2 8/9] docs: Don't mention "-mem-path" in multi-process.rst

[ThinerLogoer]

Hello,

At 2023-08-22 19:44:56, "David Hildenbrand" <david@redhat.com> wrote:
>"-mem-path" corresponds to "memory-backend-file,share=off" and,
>therefore, creates a private COW mapping of the file. For multi-proces
>QEMU, we need proper shared file-backed memory.
>
>Let's make that clearer.
>
>Signed-off-by: David Hildenbrand <david@redhat.com>
>---
> docs/devel/multi-process.rst | 5 +++--
> 1 file changed, 3 insertions(+), 2 deletions(-)
>
>diff --git a/docs/devel/multi-process.rst b/docs/devel/multi-process.rst
>index e4801751f2..4ef539c0b0 100644
>--- a/docs/devel/multi-process.rst
>+++ b/docs/devel/multi-process.rst
>@@ -409,8 +409,9 @@ the initial messages sent to the emulation process is a 
>guest memory
> table. Each entry in this table consists of a file descriptor and size
> that the emulation process can ``mmap()`` to directly access guest
> memory, similar to ``vhost_user_set_mem_table()``. Note guest memory
>-must be backed by file descriptors, such as when QEMU is given the
>-*-mem-path* command line option.
>+must be backed by shared file-backed memory, for example, using
>+*-object memory-backend-file,share=on* and setting that memory backend
>+as RAM for the machine.
> 
> IOMMU operations
> ^^^^^^^^^^^^^^^^

About "-mem-path" and "-object memory-backend-file".

I have mentioned a error message suggestion, maybe you can consider it? The
error message related to "-object memory-backend-file,id=pc.ram" is confusing,
as is shown below: 
(https://lore.kernel.org/all/2337d9f.16d6.189e8682901.Coremail.logoerthiner1@163.com/)


> 
> Wait ... I thought it should not work but it did work today. How bad am I at 
> reading
> the correct part of documentation ...
> 
> '-machine q35 -m 512M' is equivalent to '-object
> memory-backend-file,id=pc.ram,size=512M
> -machine q35,memory-backend=pc.ram',
> the latter works, and the two mentioned setup can be
> migrated from one to another.
> 
> What I was consistently trying was '-object
> memory-backend-file,id=pc.ram,size=512M -machine q35', and qemu raises an 
> error
> for this in a recent update:
> 
> ```
> qemu-system-x86_64: object name 'pc.ram' is reserved for the default RAM 
> backend, it can't be used for any other purposes. Change the object's 'id' to 
> something else
> ```
> 
> This error is misleading. Actually in this case, the error report message 
> should be more
> close to:
> ```
> object name 'pc.ram' is reserved for the default RAM backend, it can't
> be used for any other purposes. Change the object's 'id' to something
> else, or append "memory-backend=pc.ram" to -machine arguments
> ```
> 
> (I suggest rewriting the error message like this string because of the 
> confusion just now)
> 
> 
> Even though the default memory backend name is pc.ram, the
> '-machine q35,memory-backend=pc.ram' part explicitly marks that qemu
> uses a memory backend named pc.ram, rather than rely on default.
> 
> It seems that if it "rely on default" and memory-backend-file has an id
> of "pc.ram" (in x86_64 of course), it will fail.
> 
> Great. Now I will consider using a "-object
> memory-backend-file,id=pc.ram,size=512M
> -machine q35,memory-backend=pc.ram"

--

Regards,

logoerthiner